You would have your first two reductions coming in off of your main input or lay shaft going to your primary shifter shaft and then a 1:1 reduction and another reduction off of your primary shifter shaft to a secondary one.
This gear selection would give you four speeds.
Shaft 1 gear a + Shaft two 1:1 = Reduction a(1)
Shaft 1 gear b + Shaft two 1:1 = Reduction b(2)
Shaft 1 gear a + Shaft two Gear c = Reduction a * Reduction c = Reduction x(3)
Shaft 1 gear b + Shaft two Gear c = Reduction b * Reduction c = Reduction y(4)
I have these pictures for a bit of a visualization. this is something I designed a while back with the use of the gearset of an AM Gen 2 for the first stage, the shaft would be custom. It could also be done without the 1:1 but I chose to keep the original Gen2 reduction and only use a third gear to go further.
